Staple fiber webs, both single layer and composite, both pleated and unpleated, are known. Such webs are useful for a variety of purposes, such as for roofing materials, filters, insulating materials, and in apparel.
As filters in heating and cooling systems, and in some HEPA filter applications, staple fiber webs have been used for many years. The fibrous filters trap small airborne dust particles and remove the particles from a stream of air.
Fibrous filters typically function by mechanically trapping the particles. Very small particles, however, pass through the filters unless the fibers of the filter are very fine and closely packed. Such filters have the disadvantage of producing a high pressure drop, that is of creating a high resistance to air flow, through the filter.
Pleating the filters increases the filtering efficiency of the filters, without producing as high a pressure drop as is caused by more densely packing the fibers. Several disadvantages remain, however, with pleated filters. The pleating of such pleated filters tends to be dimensionally unstable unless the pleats are anchored to a supporting nonwoven or scrim. Moreover, it is difficult, if even possible, to obtain pleating which is extremely close. Therefore, much of the potential benefit of pleating is not realized.
The present invention overcomes the disadvantages of present day pleated staple fiber filters. The pleats of the webs of the invention are internally stable and do not require additional support and the pleats may be produced and maintained in extremely tight conformation.